Over-the-stern deep digging trenching plow with instrumentation for assessing the protective capabilities of a seabed trench

ABSTRACT

A seabed trenching plow has a chassis, a sled and a towing assembly. The towing assembly has a pair of wings extending laterally from each side of the chassis. The wings are aligned on an axis transverse to the chassis and adapted for connection to a towing line. The transverse axis is forward of the center of gravity of the plow and rearward of the sled, affording an over the stern releasable and retrievable trenching plow of sufficient weight and strength to excavate a three meter trench in a single pass. To assess the protective capabilities of the trench, a threshold signal indicative of a desired composition of seabed-trench soil is compared with a real-time data signal indicative of the actual to produce an alarm signal when the real-time data signal is not protective-capability compliant with the threshold signal.

BACKGROUND

This invention relates generally to seabed plows and more particularlyconcerns a deep digging over-the-stern trenching plow withinstrumentation for assessing the protective capabilities of a seabedtrench.

The present practices and equipment, typically requiring cranes andassociated heavy equipment and structures, used to release and retrievea plow from a vessel into the sea and from the sea onto the vesseltypically limit the weight of the plow to a maximum of approximately 20tons. The trenching depth and strength of known plows are compromisedaccordingly.

The depth achievable in the first trenching pass of these known 20 tontrenching plows is at best 1.4 meters. Deeper trenches can be dug bymultiple passes, but the deeper the trench and the greater the number ofpasses, the greater the forces applied to the limited strength plow.Therefore, even when multiple passes of known trenching plows are run, atrench depth of approximately 2.7 meters is the most that can beexpected. But, in many applications, trenches three meters deep may beinsufficient to protect their buried contents. Consider, for example,the impact forces that might be applied to a pipeline buried in a trenchlocated in an iceberg zone.

On the other hand, there is a plow weighing 200 tons that requires useof an A-frame or crane for launch and retrieval and can achieve a firstpass depth of 2.0 meters and a maximum total depth of 2.7 meters. Themaximum depth of 2.7 meters is dictated because the configurationrequired of the plow for launch and retrieval by A-frame or crane doesnot afford a plow of sufficient strength to withstand the forces thatwill be incurred in excavating a trench greater than 3.0 meters indepth, regardless of the number of passes used for the purpose.

Assuming that a suitable seabed trench can be excavated, the capabilityof the trench to protect pipelines, cables and other objects laid orburied in a seabed trench is a foremost concern. For example, thelikelihood that damage may be caused by icebergs and other underseaobjects drifting or otherwise moving in the vicinity of the trench is afunction of the composition of the soil in which the object is laid orburied and the depth at which the object is laid or buried in the soil.

Plow tip sensors are presently used to measure the shearing forceapplied by the tip of the plow to the seabed. Load cells are alsopresently used to measure the total tow force applied to the trenchingplow. It is presently understood that the difference between themeasured shearing and total tow forces will be generally indicative ofthe non-tip forces applied to the plow. Such information is useful tounderstanding the orientation of and the forces applied to the plowduring the trenching process but does not afford an assessment of theprotective capabilities of a trench.

The assessment is complicated because the composition of the soil maychange considerably along the path of the trench and the depth of thetrench along its path may vary somewhat from the depth expected from agiven design and adjustable configuration of the trenching plow.

It is, therefore, an object of this invention to provide a trenchingplow capable of digging trenches deeper than can be dug by knowntrenching plows. It is also an object of this invention to provide amethod for over-the-stern release and retrieval of a deep diggingtrenching plow from a vessel into the sea and from the sea onto thevessel. It is another object of this invention to provide a method andinstrumentation for assessing, on a real-time basis, the ability of atrench to protect objects laid or buried in the trench from damage bythe impact of external objects.

SUMMARY OF THE INVENTION

In accordance with the invention a seabed trenching plow has a chassis,a sled connected to a forward end of the chassis by uprights and atowing assembly. The towing assembly has a pair of wings extendinglaterally from each side of the chassis. The wings are aligned on anaxis transverse to the chassis and adapted for connection to a towingline. The transverse axis is forward of the center of gravity of theplow and rearward of the connection point of the sled uprights to thechassis.

The method of releasing the seabed plow from a deck of a vessel having astern roller includes the steps of connecting the plow to a towing lineat a point forward of a center of gravity of the plow and rearward ofthe sled uprights, causing the plow to traverse along the deck and overthe stern roller and allowing the plow to rotate by gravitation aboutthe stern roller until the plow is suspended by the towing line from thevessel aft of the stern roller.

The method of retrieving the seabed plow from the deck of the vesselincludes the steps of raising the plow from the seabed to the sternroller at the end of a towing line connected to the plow at a pointforward of a center of gravity of the plow and rearward of the sleduprights and pulling the chassis to traverse against and rotate aboutthe stern roller until the plow is resting on the deck of the vessel.

Also in accordance with the invention, a method for assessing theprotective capabilities of a seabed trench includes the steps ofgenerating a threshold signal indicative of a desired composition ofseabed-trench soil for a specific application, pulling a trenching plowhaving a plow share with a soil-analyzing tip along an intended trenchpath in the seabed, generating a real-time data signal in response tothe composition of the soil analyzed by the soil-analyzing tip along theintended trench path and comparing the real-time data signal to thethreshold signal to produce an alarm signal when the real-time datasignal is not protective-capability compliant with the threshold signal.

The step of generating the real-time data signal may include thesub-steps of measuring the force required to pull the soil-analyzingplow tip through the soil, the sleeve friction of the soil, the porepressure of the soil and the total pull force applied by the pullingmechanism to the plow and combining the measured data according to analgorithm predetermined to produce a signal indicative of thecomposition of the soil being analyzed by the soil-analyzing plow tip.

The sub-step of measuring may also include measuring the depth of thesoil-analyzing plow tip.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings in which:

FIG. 1 is a perspective view of an over-the-stern trenching plowutilizing a towing line assembly according to the invention;

FIG. 2 is a side elevation view of the over-the-stern trenching plow ofFIG. 1,

FIGS. 3A-3H are side elevation views of the over-the-stern trenchingplow of FIG. 1 in sequential transition orientations during retrieval ofthe over-the-stern trenching plow from the sea to the stern deck of atransporting/towing vessel;

FIG. 4 is a side elevation view of the plow of FIG. 1 equipped withtrench soil assessment instrumentation in accordance with the invention;and

FIG. 5 is a schematic diagram of the trench soil assessmentinstrumentation of FIG. 4.

While the invention will be described in connection with a preferredembodiment thereof, it will be understood that it is not intended tolimit the invention to those embodiments or to the details of theconstruction or arrangement of parts illustrated in the accompanyingdrawings.

DETAILED DESCRIPTION

Turning first to FIGS. 1 and 2, a trenching plow weighing as much as 100tons or more includes a chassis 10, a sled 30, a plow share 50,moldboards 60 and a towing assembly 80.

The chassis 10 shown has three elongated vertical plates 11 spaced bytransverse vertical reinforcing plates 13 and extending from a noseplate 15 to an end plate 17. The bottom of the chassis 10 lies in foreand aft horizontal planes 19 and 21 with an intermediate plane 23 angleddownwardly fore to aft. The chassis 10 has a convex nose 25 beginning atthe top edge of the nose plate 15 and transitioning into a downwardlyangled midsection 27 followed by a horizontal end section 29 extendingto the top edge of the end plate 17.

The sled 30 is mounted on the chassis 10 below its nose 25. Uprights 31are pivotally pinned between the sled skids 33 and brackets 35 mountedon the underside of the nose 25 and a reinforcing strut 37 is pivotallypinned between the uprights 31 and the angled midsection 27 of thechassis 10. The uprights 31 and reinforcing strut 37 are apertured andpinned to permit adjustment of the vertical distance between the chassisnose 25 and the angle of the uprights 31 with respect to vertical. Thesled uprights 31 are pinned to the chassis nose brackets 35 on a commonaxis 39.

The plow share 40 as shown is mounted in any known manner against thebottom of the horizontal end portion 29 of the chassis 10, as shownunder the aft section 17 of the chassis 10, with the tip 41 of the plowextending forward to approximately a point below the junction of theangled midsection 27 of the chassis 10 with the horizontal end portion29 of the chassis 10. The plow share 40 is in shape generally similar toknown plow shares. However, its tip 41 is considerably further below itschassis 10 than the tips of known plow shares, the present plow tip 41being as much as three meters below the chassis 10 in comparison toknown plow tips which are no more than 1.4 meters below their chassis.Its weight is significantly greater than the weight of most known plowshares, the present plow share 40 weighing as much as 100 tons or morein air in comparison to known plow shares which weigh no more than 40tons in air. Its width may be, but is not necessarily, wider than thewidth of known plow shares, the present plow share 40 being as much asnine meters wide in comparison to known plow shares which are no morethan 4.2 meters wide.

The moldboards 50 are mounted in any known manner against the outeraft-most faces of the outer vertical elongated plates 11. The moldboards50 are generally similar to known moldboards, though their weight maybe, but is not necessarily, significantly greater than the weight ofknown moldboards, the present moldboards 50 weighing as much as ten tonsin comparison to known moldboards which weigh no more than two tons.Preferably, each of the moldboards 50 is divided into proximal anddistal sections 51 and 53 joined by hinge pins 55 at angled-cut ends 57.Wedges 59 can be inserted above or below the hinge pins 55 so that thebottom of the moldboard distal sections 53 can be locked in either ahorizontal or upwardly angled condition relative to the bottom of theproximal sections 51 of the moldboards 50. As seen in FIG. 1, themoldboards 50 are preferably provided with rollers 61 so as to reducefriction when the moldboards 50 traverse the deck of a vessel and aconnecting frame 63 providing reinforcement between the distal sections53 of the moldboards 50.

As shown, a towing assembly 70 is located on the downwardly angledmidsection 27 of the chassis 10 aft of the connection point of the sleduprights 31 to the chassis nose 25. In the embodiment shown, the towingassembly 70 includes wings 71 mounted against the outer faces of theouter vertical elongated plates 11. Each wing 71 carries mounting rings73 aligned on a common axis 75 to facilitate connection, perhaps in aclevis fashion, to a tow line (not shown).

Looking at FIG. 2, the key parameters of the present trenching plow arethe locations of its center of gravity 81, of the connecting axis 39between the sled uprights 31 and the chassis nose brackets 35 and thecommon axis 75 of the towing assembly mounting rings 73. In accordancewith the invention, the common axis 75 of the towing assembly mountingrings 73 must fall between the sled upright connecting axis 39 and theplow center of gravity 81.

Preferably, and as shown, the center of gravity 81 of the present plow,which weighs as much as 100 tons or more, is approximately 15 meters aftof the sled upright connecting axis 39 and the common axis 75 of thetowing assembly mounting rings 73 is approximately midway between thecenter of gravity 81 and the sled upright connecting axis 39. Incomparison, known trenching plows have a center of gravity approximately5-6 meters aft of the nose of the plow, about ⅓ to ⅖ the distance of thepresent plow, and a tow line connection point forward of the uprights.Therefore, the present plow results in a moment as much as 12.5 to 15times that of known plows.

In practice, the towing line connection assembly 70 can be located toposition the common axis 75 of the towing assembly mounting rings 73anywhere between the center of gravity 81 and the sled uprightconnecting axis 39. However, the closer the common axis 75 of themounting rings 73 is to the center of gravity 81 the better, so long asit is forward of the center of gravity 81.

The configuration and weight of the chassis 10, sled 30, plow share 50,moldboards 50 and towing assembly 70 are coordinated to position thecenter of gravity 81 of the plow at a location affording a resultingmoment suitable to a given 20 to 100 ton or more trenching plowapplication.

Looking at FIGS. 3A-3H, assume a plow weight of 96 tons and a center ofgravity 81 approximately 15 meters aft of the sled upright connectingaxis 39. The transition of the over-the-stern trenching plow across thestern roller R of a transporting/towing vessel V during retrieval fromthe sea W is sequentially shown from a point P₁ of first contact withthe roller R to a point P₈ at which the plow has entirely traversed theroller R and is at rest on the deck D of the vessel V.

Beginning with FIG. 3A, the plow has been retrieved at the end of awinch driven tow line L to the point P₁ with the plow oriented forcontact between the roller R and the top surface of the nose 25. Thetowline L remains turned on the roller R. In this orientation, themoment of the plow about the roller R is near minimal.

As is seen in FIG. 3B, the plow has been further retrieved to a point P₂at which the apex of the convex nose 25 is in contact with the roller R,the towline L remains turned on the roller R and the center of gravity81 of the plow has rotated the slightly astern of its position in FIG.3A. In this orientation, because of the convex structure of the nose 25and the sternward shift of the center of gravity 81, the moment of theplow about the roller R is greater but still near minimal.

As is seen in FIG. 3C, the plow has been further retrieved to a point P₃at which the common axis 75 of the towing assembly mounting rings 73 isabove the contact point P₃ and below the high point of the roller R, sothat the towline L is slightly turned on the roller R. Also, the contactpoint P₃ has shifted to the downwardly angled midsection 27 of thechassis 10. The center of gravity 81 of the plow has rotationallyshifted further slightly sternward but very little net shift of thecenter of gravity 81 has occurred because of the angled midsection 27 ofthe chassis 10. Therefore, in this orientation, the moment of the plowabout the roller R is substantially the same as in FIG. 3B, which isstill near minimal.

As is seen in FIG. 3D, the plow has been further retrieved to a point P₄at the junction of the downwardly angled midsection 27 and thehorizontal end section 29 of the chassis 10. The common axis 75 of thetowing assembly mounting rings 73 has shifted above the roller R. Thetowline L no longer contacts the roller R and has levered the chassis 10at the fulcrum point P₄ to shift the center of gravity 81 of the chassis10 to approximately 2.334 meters 83 astern of the fulcrum point P₄,creating a total moment of 224.1 metric ton-meters.

As is seen in FIG. 3E, the continued pull of the towline L has causedthe horizontal end section 29 of the chassis 10 to advance slightly onthe roller R and has significantly levered the chassis 10 at the fulcrumpoint P₅ to further shift the center of gravity 81 of the chassis 10 toapproximately 3.768 meters 85 astern of the fulcrum point P₅ creating atotal moment of 362 metric ton-meters.

As is seen in FIG. 3F, further continued pull of the towline L hascaused the horizontal end section 29 of the chassis 10 to advance moresignificantly on the roller R, levering the chassis 10 at the fulcrumpoint P₆ to further shift the center of gravity 81 of the chassis 10 toapproximately 4.518 meters 87 astern of the fulcrum point P₆, creating atotal moment of 433.9 metric ton-meters, the maximum total moment of theretrieval process.

As is seen in FIG. 3G, further continued pull of the towline L hascaused the chassis 10 to advance until the center of gravity 81 of thechassis 10 is substantially but not quite directly above the contactpoint P₇, reducing the total moment of the plow about the roller R onceagain to near minimal.

Finally, looking at FIG. 3H, further continued pull of the towline L hascaused the chassis 10 to advance until the plow is entirely forward ofthe stern roller R and the plow is resting on the deck D of the vesselV.

The release of the plow from the deck D of the vessel V into the sea Sis essentially the reverse of the retrieval process illustrated in FIGS.3A-3H, except that independent winch lines are used to pull the plow inthe opposite direction across the stern roller R, as by ablock-and-tackle assembly, against the tension of the towing line L.

Turning now to FIG. 4, in order to assess the protective capabilities ofa seabed trench dug by a trenching plow such as the plow of FIG. 1, theplow share 40 is equipped with a soil-analyzing tip 41. Thesoil-analyzing tip 41 includes load pins 43, a pressure sensor 45 and afriction sensor 47. The load pins 43 measure the tip reaction force 83which is the force required to pull the soil-analyzing plow tip 41through the soil. For example, the plow design may anticipate a tipreaction force 83 up to 650 tons. The pressure sensor 45 measures thepore pressure 85 of the soil passing under the plow tip 41. The frictionsensor 47 measures the sleeve friction 87 of the soil passing under theplow tip 41. A load cell 49 is located on the plow or elsewhere in aposition suitable to measure the total pull force 89 applied to the plowvia the tow line L by its pulling mechanism, such as one or more vesselsor winches.

For example, the plow design may anticipate that a total pull force 89on the plow will be in a range of 200 to 250 tons. Since the total pullforce 89 is measured and the offsetting tip reaction, sleeve andfriction forces 83, 85 87 are also measured, the forces exerted on theplow share 40 between the plow tip 41 and the bottom of the chassis 10,a distance in the range of 3 meters, is calculable.

Turning to FIG. 5, data transfer units 91 powered by batteries 93 arecable-connected to the sensors 43, 45 and 47 in the plow tip 41 andcollect data to be received by remote data receiving units 95 which may,for example, be located on remote operated vehicles 97 in communicationwith a vehicle controller 99, a plow controller 101 and a GPS device103. The data transfer units 91 may, for example, be SENTOOTH 100® datatransfer units.

In operation, the method for assessing the protective capabilities of aseabed trench includes the steps of generating a threshold signalindicative of a desired composition of seabed-trench soil for a specificapplication. The trenching plow, which has a plow share 40 with asoil-analyzing tip 41, is pulled along an intended trench path in theseabed. As the plow is pulled along the intended path, a real-time datasignal is generated in response to the composition of the soil analyzedby the soil-analyzing tip 41. The data signal is herein identified asbeing a real-time signal because the amplitude of the signal iscoordinated to the position of the plow along the length of the trench.If and when the soil is backfilled into the trench to further increasethe protective capability of the trench, within reasonable limitations,the backfilled soil will be the soil that was excavated and analyzedduring trenching, so that the data signal substantially accuratelyindicates the varying composition of the soil along the backfilledtrench. If the trench is not backfilled, the data signal will even moreclosely indicate the varying composition of the soil defining thetrench.

The real-time data signal is then compared to the threshold signal toproduce an alarm signal when the real-time data signal is notprotective-capability compliant with the threshold signal.

The step of generating a real-time data signal may include twosub-steps. The force required to pull the soil-analyzing plow tipthrough the soil, the sleeve friction of the soil, the pore pressure ofthe soil and the total pull force applied by the pulling mechanism tothe plow are all measured as the plow is pulled along the intendedtrench path. The measured data is combined according to an algorithmpredetermined to produce a signal indicative of the composition of thesoil being analyzed by the soil-analyzing plow tip. The algorithm may bestandardized or unique to a given application so as to weigh themeasured data according to the desired predominance of its importance ina given protective capability analysis.

The sub-step of measuring may also include measuring the depth of thesoil-analyzing plow tip for inclusion in the measured data beingcombined according to the algorithm so as to enable accounting for depthvariations that may occur along the length of the trench.

By way of example, a suitable algorithm might weigh the plow tipreaction force, the sleeve friction of the soil, the pore pressure ofthe soil, the total pull force applied to the plow and the deviation ofthe depth of the trench from a predetermined depth as 70%, 10%, 10%, 5%and 5%, respectively.

Thus, it is apparent that there has been provided, in accordance withthe invention, an improved over-the-stern trenching plow and a method ofreleasing and retrieving the plow from the vessel into the sea and fromthe sea onto the vessel and a method and instrumentation for assessingthe protective capabilities of a seabed trench that fully satisfy theobjects, aims and advantages set forth above. While the invention hasbeen described in conjunction with a specific embodiment thereof, it isevident that many alternatives, modifications and variations will beapparent to those skilled in the art and in light of the foregoingdescription. Accordingly, it is intended to embrace all suchalternatives, modifications and variations as fall within the spirit ofthe appended claims.

What is claimed is:
 1. For use in assessing the protective capabilitiesof a seabed trench, a method comprising the steps of: generating athreshold signal indicative of a desired composition of seabed-trenchsoil for a specific application; pulling a trenching plow having a plowshare with a soil-analyzing tip along an intended trench path in theseabed; generating a real-time data signal in response to thecomposition of the soil analyzed by the soil-analyzing tip along theintended trench path; and comparing the real-time data signal to thethreshold signal to produce an alarm signal when the real-time datasignal is not compliant with the threshold signal.
 2. A method accordingto claim 1, said step of generating a real-time data signal comprisingthe sub-steps of: measuring the force required to pull thesoil-analyzing plow tip through the soil, the sleeve friction of thesoil, the pore pressure of the soil and the total pull force applied bythe pulling mechanism to the plow; and combining the measured dataaccording to an algorithm predetermined to produce a signal indicativeof the composition of the soil being analyzed by the soil-analyzing plowtip.
 3. A method according to claim 2, said sub-step of measuringfurther comprising measuring the depth of the soil-analyzing plow tip.